Target State Optimization with Density Functional Theory for Computing K-Edge X-ray Absorption Spectra.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Time-Dependent Density Functional Theory (TDDFT) often underestimates core excitation energies in molecular X-ray Absorption Spectra (XAS) by over 10 eV.
• Accurate prediction of core excitations is crucial for understanding molecular electronic structure.

Purpose of the Study:

• To evaluate the performance of Target State Optimization Density Functional Theory (TSO-DFT) for predicting molecular K-edge X-ray Absorption Spectra (XAS).
• To demonstrate TSO-DFT's capability in capturing orbital relaxation effects for improved accuracy in core excitation energy predictions.

Main Methods:

• Application of TSO-DFT to calculate K-edge XAS for various molecules, including CO2, N2O, carbonyl compounds, and radicals.
• Prediction of angle-dependent XAS for porphyrin and polarized XAS for the uranyl ion using TSO-DFT.

Main Results:

• TSO-DFT achieves sub-electronvolt accuracy in predicting core excitation energies, significantly improving upon TDDFT.
• The method successfully predicts main spectral features and core excitation energies for diverse molecular systems.
• Calculated XAS spectra show excellent agreement with experimental data, providing insights into electronic structure changes.

Conclusions:

• TSO-DFT is a highly accurate and promising computational method for the study of molecular X-ray Absorption Spectra.
• The TSO-DFT method is accessible through the free Qbics software package.